On predicting chip morphology and phase transformation in hard machining

Abstract

A finite element model is developed to predict the chip formation and phase transformation in orthogonal machining of hardened AISI 52100 steel (62HRC) using Polycristalline Cubic Boron Nitride (PCBN) tools. The model mainly includes a chip separation criterion based on critical equivalent plastic strain; a Coulomb's law for the friction at the tool/chip interface; a material constitutive relation of velocity-modified temperature; a thermal analysis incorporating the heat dissipated from inelastic deformation energy and friction; and an annealing effect model, in which the work hardening effect may be lost or re-accumulate depending on material temperature. This fully coupled thermalmechanical finite element analysis accurately simulates the formation of segmental chips and predicts the phase transformation on the chips, as verified by experiment. It is found that high temperatures around the secondary shear zone causes fast re-austenitization and martensite transformation, while other parts of the chips retain the original tempered martensitic structure.

Dislocation Density and Grain Size Evolution in the Machining of Al6061-T6 Alloys

“…Based on previous research [3][4][5][9][10][11]14,15,[19][20][21], this study aims to propose an iterated microstructure sensitive FE model to address the microstructure evolution mechanism in terms of dislo cation density and grain size alteration in the machining of A16061-T6 alloys. This iterated FE model incorporates dislocation density-based microstructure evolution equations in the form of subroutines (V uhard) written in fortran, which has potentials to predict dislocation densities and grain sizes in machined surface.…”

“…This model incorporated the effects of stress and strain on phase trans formation, volume expansion and SPD. Shi and Liu [3][4][5]14] pro posed a methodology to decompose thermal and mechanical effects on the microstructure in the chip and hard turned surface of AISI 52100. Shi et al [15] investigated the effects of machin ing parameters on the thickness of white layer in hard turning AISI 52100.…”

Model Describing Material-Dependent Deformation Behavior in High-Velocity Metal Forming Processes

“…This will be discussed later in greater detail. For the QT material, suitable parameter choices were found to be t α = [−6.83 × 10 −5 1/K, −5.07 × 10 −5 1/K, 0.017], m α = [0.454, 0.026] and θ = 770 K. Note that a similar drop in initial yield stress is reported by Shi and Liu (2006) for an equivalent material, heat treated to a hardness of 62 HRC. The high-temperature response of the QT material can be affected by the sensitivity of its martensitic structure to elevated temperatures.…”

“…During deformation, the internal temperature of the QT-material can rise to well above the austenitization temperature of approximately 750 • C. This heating, in combination with subsequent rapid and localized cooling, allows reaustenitization and formation of new martensite in the material to take place, cf. Shi and Liu (2006). These microstructural changes commonly appear in connection with localization of the deformation and the formation of so-called white layers or etching bands.…”

Simulation and experimental studies to verify the effect of cutting parameters on chip shrinkage coefficient and cutting forces in machining of A6061 aluminum alloy

“…Based on the J-C model, the fracture occurs only when the material is plastically flowing. 4 Several authors have employed different fracture models for simulation of material cutting process such as constant fracture strain model, 5 Wilkins model, 6 modified Cockcroft-Latham model, 7,8 or power law. 9 In general, all the models can give consistent predictions in cutting forces, chip thickness, and shear angle with adequate accuracy.…”